It is known that even in the adult human, bone can be subject to turnover. In certain locations, such as the internal auditory capsule, there is apparently no turnover after the organ is formed. In other locations, particularly in the central skeletal axis, the turnover appears to continue during adulthood. Bone turnover occurs on the surface of the existing bone matrix, which is composed of protein (mainly collagen) and minerals. Bone turnover is initiated with the destruction of bone matrix by osteoclasts. An osteoclast is a multinucleated cell which secretes acid and proteolytic enzymes leading to the lysis of the collagen matrix protein and the release of minerals into the extracellular fluid compartment. Following this initial phase of bone destruction, or resorptive phase, formation of new bone protein matrix sets in. New bone proteins are deposited, and sometime later, minerals begin to be incorporated into the newly formed matrix. The formation of bone matrix and its subsequent mineralization are functions of osteoblasts, which are mononucleated cells. The formation phase is often followed by a period of inactivity (1,2). Resorption appears to be tightly coupled with formation (3) in vivo. Bone turnover is thus a succession of events, the location of which is known as the Bone Metabolism Unit or the BMU. Osteoblasts and osteoclasts, the putative mediators of bone turnover are thought to belong to two distinct cell lineages. These two cell types are not preformed cells, but they differentiate from their precursors through cell activation (4,5,6).
Bone matrix can either be maintained by a total cessation of bone turnover, as for the bone of the internal auditory capsule, or by a balance between formation and resorption. In many studies on skeletal changes in relation to age, a gain in the total body bone volume is observed during the growth period and the skeletal mass reaches a maximum at early adulthood. This gain is followed by a fall in bone volume as age advances. In females, a phase of more rapid bone loss often occurs during the perimenopausal period before a slower steadier phase. For this reason, bone loss in the female tends to be more severe than in the male. An understanding of bone balance in the BMU may thus be critical to understanding the pathogenesis of skeletal aging. In any case, mechanisms controlling bone turnover are complex and are not well understood at this time. The complexity of the control mechanisms has resulted in a variety of approaches to reducing bone loss.
Generally speaking, bone turnover can be regulated at two different stages. It can be regulated at the stage of the activation of precursor cells. Regulators of cellular activation can control not only the number of active BMU in the skeleton, but possibly also the number of osteoclasts and osteoblasts in an individual BMU. Alternatively, bone turnover can be regulated at the level of differentiated bone cells. The complexity of the bone cell system makes the separate study of these two levels of regulation difficult (3).
Regulators of bone cells appear to fall into two categories. The first of these interacts with specific receptors on cell membranes. One class of these regulators acts through the adenylate cyclase system with the generation of intra-cellular cyclic AMP as a second messenger acting on the protein kinase K system. Parathyroid hormone (PFM) and calcitonin (CT) belong to this class (7). A second class also interacts with a membrane receptor and results in the intracellular release of a molecule derived from phosphoinositides which in turn leads to an increase in intracellular calcium and activation of Kinase C. A third class involves interaction of the regulator with a cell surface receptor, but the second signal is generated by the receptor molecule itself with the subsequent activation of tyrosine Kinase. Many of the growth factors appear to act in this way (8-15). The second category of regulator does not interact with a cell membrane receptor, but can cross the cell membrane to bind with a cytosolic receptor. The regulator is then transported across the nuclear membrane by the cytosolic receptor to interact with the DNA resulting in increased transcription of specific genes. Steroid hormones, including vitamin D, appear to act in this manner (16).
Many hormones stimulate the proliferation of osteoclasts. These include 1,25(OH).sub.2 D, PTH and prostaglandins. PTH and 1,25(OH).sub.2 D receptors in osteoclasts have apparently not yet been identified These two hormones seem to have no effect on osteoclasts in culture. However, when osteoclasts are co-cultured with osteoblast-like cell lines, PTH and 1,25(OH).sub.2 D stimulate the proliferation of osteoclasts. IL-1 and TNF appear to act in a similar way as PTH and 1,25(OH).sub.2 D. Other growth factors, like EGF, TFG and PDGF appear to stimulate osteoclasts through increased production of PGE. Calcitonin and corticosteroids are known osteoclast inhibitors along with chemicals such as diphosphonates.
It is currently believed that interleukin 1 may stimulate collagen and non-collagen bone protein and DNA synthesis. The effect on bone protein synthesis is blocked by indomethacin, suggesting that this action of IL-1 is mediated through PGE. Indomethacin seems to have no effect on the IL-1 effect on osteoblast DNA synthesis. In culture studies on osteoblast-like cell lines suggest that some locally produced growth factors stimulate DNA and collagen synthesis. In bone cell culture, PTH or Vitamin D suppresses collagen synthesis. This in vitro effect of PTH contrasts with the in vivo effect observed in human subjects and experimental animals. It has been demonstrated in rats and in human hyperparathyroid patients that PTH can stimulate the deposition of mineralized bone matrix. Preliminary clinical trial studies on the efficacy of the PFM 1-34 amino acid fragment in the treatment of osteoporosis indicate that this PTH fragment can increase the trabecular volume. The reason for this discrepancy is not yet fully explained.
Parathyroid hormone is a peptide of 84 amino acids in its mature form. Initially translated pre-pro-parathyroid hormone is much larger, the pre sequence being a signal sequence which is cleaved when the peptide enters the rough endoplasmic reticulum. In the golgi apparatus, the pro-sequence is cleaved off leaving the intact mature hormone packaged in the secretory granule. It appears that regulation of the rate of secretion is governed not so much by the rate of production of the intracellular peptide, but in the rate of intracellular destruction and in the rate of secretion. Intracellularly, the mature peptide is truncated at both the amino and the carboxyl termini. The truncated peptide may be secreted into the circulation as an inactive fragment. The secretion of the mature peptide can be stimulated by a drop in the extracellular calcium concentration. An elevated serum calcium concentration on the other hand appears to suppress the secretion of PTH. Once in circulation, the mature peptide is rapidly cleaved in the liver at many sites of the molecule including the region of the 38 amino acid residue. The smaller fragment at the amino terminal end, which includes the first 34 amino acids, carries the full known biological activity in terms of its action on the kidney, the intestine and the bone. It also binds fully to the cell membrane receptor to stimulate cAMP production. The level of the 1-38 fragment in the serum is normally unmeasurable indicating that it has a short circulatory life. The larger inactive carboxyl terminal fragment has a relatively long half life and carries the highest proportion of the immunoreactive PTH in the circulatory system. All fragments in circulation are eventually destroyed in the kidney and the liver. One of the renal mechanisms for ridding the circulating inactive PTH fragments is glomerular filtration (17).
PTH participates in calcium and skeletal homeostasis. PTH stimulates the tubular resorption of calcium by the kidney and inhibits the reabsorption of phosphate and bicarbonate by the proximal renal tubules. A second effect of PTH on the kidney is the stimulation of 1,25(OH).sub.2 D production. This vitamin D metabolite is an in vivo stimulator of osteoclasts as well as an enhancer of intestinal calcium absorption. The increase in calcium absorption by the intestine following PTH stimulation is mediated by this vitamin D metabolite. In vivo, PTH stimulates osteoclastic bone resorption with the release of calcium into the circulation. PTH also causes proliferation of osteoblasts (18). In many cases of hyperparathyroidism there is a skeletal loss. However, an increase in spinal density has been reported in some cases of primary hyperparathyroidism (19,20,21) as well as in secondary hyperparathyroidism complicating renal failure. Kalu and Walker have observed that chronic administration of low doses of parathyroid extract led to sclerosis of bone in the rat (22). Tam et al studied the effect of low calcium diet on the bone mineral apposition rate in the rat by tetracycline labelling and found that despite the loss of bone due to increase in bone resorption histologically (as a result of secondary hyperparathyroidism), the bone mineral apposition rate was increased (23). It was also found that the bone mineral apposition rate was increased in 23 human patients with mild primary hyperparathyroidism (24). After successful removal of parathyroid adenoma from four of the patients, the rate returned to the level observed in control subjects. There has also been found to be a dose dependent stimulation of the mineral apposition rate by PTH. The potency of the 1-34 fragment and the intact PTH hormone appears to be about the same on a molar basis. This is consistent with the 1-34 fragment of the PTH molecule carrying the biological activity of the intact hormone. It has also been observed that the end result of the administration of PTH on skeletal homeostasis depends on how the hormone is administered. For the same daily dose, the bone volume shows a dose dependent increase if the daily dose of the hormone is given as one single injection. However, when the same daily dose is administered by continuous infusion with a subcutaneous miniosmotic pump, the result is bone loss. Intermittent injection causes practically no effect on the serum calcium levels whereas infusion causes a dose dependent increase in the serum calcium. The effects of PTH administered by these two routes on bone mineral apposition rate as measured by tetracycline labelling are the same. What accounts for this differential effect is not understood (25).
Given the general understanding of bone growth and its regulation, various approaches to treatment of diseases involving reduction of bone mass and accompanying disorders are exemplified in the patent literature. For example, PCT Patent Application No. 9215615 published Sep. 17, 1992 describes a protein derived from a porcine pancreas which acts to depress serum calcium levels for treatment of bone disorders that cause elevation of serum calcium levels. European Patent Application No. 504938 published Sep. 23, 1992 describes the use of di- or tripeptides which inhibit cysteine protease in the treatment of bone diseases. PCT Patent Application No. 9214481 published Sep. 3, 1992 discloses a composition for inducing bone growth, the composition containing activin and bone morphogenic protein. European Patent Application No. 499242 published Aug. 19, 1992 describes the use of cell growth factor compositions thought to be useful in bone diseases involving bone mass reduction because they cause osteoblast proliferation. PCT Patent Application No. 4039656 published Jun. 17, 1992 describes a drug containing the human N-terminal PTH fragment 1-37. European Patent Application No. 451867 published Sep. 16, 1991 describes parathyroid hormone peptide antagonists for treating dysbolism associated with calcium or phosphoric acid, such as osteoporosis.
The relatively short half life of PTH in the blood serum and the relatively lengthy effect of intermittent PTH injection led the present investigator to the hypothesis that PTH may in some way lead to induction of a second factor into the circulatory system. The presence of such a second factor in blood serum of rats and of humans has thus been investigated.
It has been found possible to isolate from rat blood serum a polypeptide substance which, upon administration to rats incapable of producing PTH (parathyroidectomized rats), produces an increase in the observed bone mineral apposition rate. The desired polypeptide can be obtained from the isolated polypeptides by removing a polypeptide having a PI of about 9. It has further been observed that the bone apposition rate increases with the dose of the isolated substance administered, at least over the dose range and time period studied. The substance has been isolated in two forms, a first larger polypeptide having a molecular weight about twice that of a second smaller polypeptide. The first eleven amino acids of the sequence of the smaller polypeptide have been determined to be Gly Pro Gly Gly Ala Gly Glu Thr Lys Pro Ile (SEQ ID NO:1). The first seven amino acids of the larger polypeptide have been determined to be Gly Pro Gly Gly Ala Gly Glu (SEQ ID NO:2). The similarity of these two NH.sub.2 -terminal sequences has led to the proposition that the larger polypeptide might be the dimer of the first.
A nucleic acid probe, based on the amino acid sequence of the rat peptide has been synthesized and used to screen a human liver cDNA fetal library in order to isolate a human nucleic acid sequence coding for a human bone apposition polypeptide. A polypeptide was thus chemically synthesized according to the sequence Gly Ile Gly Lys Arg Thr Asn Glu His Thr Ala Asp Cys Lys Be Lys Pro Asn Thr Leu His Lys Lys Ala Ala Glu Thr Leu Met Val Leu Asp Gln Asn Gin Pro (SEQ ID NO:11). It has been observed that the bone apposition rate in intact rats increases in a dose dependent fashion upon administration of this chemically synthesized compound. Reduced bone growth, normally observed for ovariectoniized rats, was observed not to occur in rats after being administered with the polypeptide over a four week period beginning two weeks after ovariectomization. Bone calcium density was found to be maintained in ovariectoneeed rats administered with the polypeptide over an eight week period beginning eight weeks after ovariectomization.
It is thought possible that the active polypeptide is a dimer of the foregoing sequence, there being evidence of significant dimer formation, presumably due to a disulfide bridge between two polypeptides having the sequence shown.
A modified form of the polypeptide containing a cys.fwdarw.ala substitution was thus synthesized: Gly Ile Gly Lys Arg Thr Asn Glu His Thr Ala Asp Ala Lys Ile Lys Pro Asn Thr Leu His Lys Lys Ala Ala Glu Thr Leu Met Val Leu Asp Gln Asn Gln Pro (SEQ ID NO:13). Some of the bone stimulatory effects of the normal polypeptide were found for the modified polypeptide.
The bone mineral apposition rate in rats administered with rabbit antibodies to the normal polypeptide (SEQ ID NO:11) was found to be suppressed. The suppression was found to be attenuated in rats administered with both the normal polypeptide and antibodies to same.